This invention relates to an improvement in liquid cooled stator windings of rotary electric machines, and more particularly to a liquid cooled winding of the kind above described in which a liquid coolant header is fixedly mounted by heat welding on the stator bar ends.
The capacity of a modern rotary electric machine such as turbogenerator and water-wheel generator has been increased to such an extent that indirect cooling of the stator winding by circulation of liquid coolant over the insulator covering of the winding is no longer effective in sufficiently removing heat generated by the electrical current flowing through the stator bars. In the modern rotary electric machines, therefore, it is the present tendency to provide cooling passages extending through electrically conductive stator bars thereby furnishing direct cooling by the circulation of liquid coolant such as water or oil. In order to circulate the liquid coolant through the hollow stator bars, headers are provided on suitable portion of the stator bars. These headers are generally mounted on the ends of bars extending outwardly from the stator core in a generally axial direction so that the mounting work thereof can be easily done.
FIGS. 4 and 5 show one form of the bar end arrangement, that is, header arrangement employed heretofore in a rotary electric machine of the kind above described, and are longitudinal sectional views of part of the stator. Referring to FIGS. 4 and 5, a liquid coolant header 5 connects the ends of stator bars 3 and 4 of a stator winding 2 extending outwardly from a stator core 1 in a generally axial direction. The stator bars 3 and 4 are made up of a plurality of hollow conductive elements 20. This header 5 itself is made of an electrical conductive material in order to electrically connect the stator bars 3 and 4 and is fluidly connected by a conduit 6 to a source of liquid coolant, such as a pump or cooler (not shown). The connection between the header 5 of electrical conductive material and the electrically conductive stator bars 3 and 4 must be such that they are sufficiently jointed together both electrically and fluidly, and what is more important is that they must be completely fluid-tight at the joint so as to prevent even slight leakage of liquid coolant. As is well known, periodic inspection on a rotary electric machine, for example, a turbogenerator is carried out only once every several years of operation and hence once it is placed in service, the turbogenerator continues during this long period of time to operate. Therefore, slight leakage or exudation of liquid coolant from the joint will result in attachment of ambient dust to the joint, and the dust thus accumulating which is rather electrically conductive will adversely affect the electrical insulation and will lead to a serious accident such as storing in a worst case.
Careful attention must therefore be directed to leakage of liquid coolant from the cooling passages in the rotary electric machine. Extremely high reliability is thus required for the stator bars. An elaborate experiment made by the inventors in an effort to improve the reliability has proved the following fact. The stator bars are generally jointed to the header by means of brazing. During the heating for the purpose of brazing, however, all of a plurality of conductive elements constituting each stator bar are not necessarily uniformly heated. Especially, in the case of high-frequency induction heating, only some of the conductive element may be heated up to a higher temperature than the other elements depending on the arrangement of the heating coil. As a result, the conductive element or elements heated to higher temperature will be subject to more thermal elongation than the others in an outward direction or away from the stator core and then fixed to the header in the state as it is. In such a case, a residual stress occurs at the connection point of the conductive element or elements subjected to the thermal elongation after brazing. In the rotary electric machine in which violent vibrations occur during operation, the individual conductive elements are not always elongated in an equal amount, and this will impart a further stress to the joint, thereby increasing the overall degree of unbalance at the joint. This additional stress due to the unbalance is combined with the residual stress and vibrations and gives rise to creation of local cracks which lead to undesired leakage of liquid coolant at the joint.